1. Field of the Invention
The present invention relates to a local heating apparatus for sheet glass and, more particularly, to a local heating apparatus suitably used in an apparatus for shaping window glass for a motor vehicle.
2. Description of the Prior Art
Sheet glass which is moderately curved as a whole and has both acutely curved side portions is frequently used as a windshield or rear window glass for a motor vehicle.
The sheet glass having such bent portions is obtained such that sheet glass is softened in a heating furnace and is curved in a bending mold by its own weight or curved by press bending molds consisting of female and male molds. A local heating means is arranged in the heating furnace to forcibly heat a glass plate portion having a smaller radius of curvature.
U.S. Pat. No. 4,441,907 by Nitschke describes that a elongated local heater is located above a bending line of sheet glass while sheet glass is fed at a constant conveyance speed in a heating furnace. The local heater is mounted on a carriage moved in a direction parallel to a conveyance direction of sheet glass and continuously heats a predetermined portion of glass. When the carriage reaches a downstream end of the heating furnace, it returns to an upstream end add performs the next heating cycle of another glass plate.
In the heating apparatus described in the above U.S. patent, the local heater must be moved together with sheet glass, and the structure of the heating furnace is complicated. In addition, a large opening must be formed in the heating furnace to move the heater, and a heat loss is undesirably increased. Furthermore, since the carriage of the local heater returns to the home position after local heating of one glass plate is completed, a shaping cycle is undesirably prolonged.
When the type of sheet glass is changed, the position of the local heater must be changed to align with a heating line. When various types of sheet glass are manufactured in a small volume, productivity efficiency is greatly degraded.
U.S. Pat. No. 4,726,832 by Kajii et al. describes an apparatus capable of coping with shapes of bending lines of various types of sheet glass by digitally controlling the position of a local heater and updating control data. The local heater of this apparatus is a spot-like gas burner which is fixed in the conveyance direction of sheet glass and can be position-controlled by a digitally controlled actuator in a widthwise direction of the heating furnace. Position control of the local heater is performed in synchronism with the conveyance speed of sheet glass in the heating furnace.
Synchronization control of the heater position is performed by digital tracking control. That is, basically, output pulses from a pulse generator arranged in a drive shaft of a conveyor in the heating furnace are counted to detect a current position of glass in the longitudinal direction of the heating furnace. Coordinate data of the heater in the heating furnace in the widthwise direction is read out from a memory area at an address corresponding to the current position. An actuator such as a servo motor is driven on the basis of the readout data to control the position of the heater. Coordinate data in the widthwise direction of the heating furnace which are calculated for a large number of control points on a glass bending line are stored by using the longitudinal coordinate positions as addresses.
According to the above digital control, an actual locus of the heater which is formed on the glass surface is offset from the glass bending line, and a heating temperature of a bent portion does not undesirably reach an optimal point.
The offset of the heating locus from the target heating position is caused by response delay of a heating positioning system and quantization errors inherent to digital control. In particular, the heating locus is not represented by a straight line but by a zig-zag or stepwise pattern having a step between adjacent control points.
In order to solve the above problem, the number of control points assigned with tracking addresses in the longitudinal direction of the furnace must be increased to effectively improve positioning precision. However, when the number of control points is increased, the memory capacity of position control data is increased, and high-speed response cannot be achieved.